Image analyzing system



Allg 26, 1941- P. T. FARNswoRTH 2,254,140

IMAGE ANALYZING SYSTEM Filed Sept. lO, 1938 Fgl.

\ una E/ Ecmo/v scANN/NG /MAGE BEAM /NsuL'AT/o/v METAL INVENTOR,

PH/LO 7T FARNS WORTH.

Patented Aug. 26, 1941 Ill/IAGE ANALYZIN G SYSTEM Philo T. Farnsworth,Springfield Township, Montgomery County, Pa., assignor, by mesneassignments, to Farnsworth Television & Radio Corporation, Dover, Del.,a corporation of Dela- Application September 10, 1938, Serial No.229,350

(Cl. TIS- 7.2)

7 Claims.

My invention relates to limage analyzing systems, and more particularlyto a system employing image analysis electron tubes wherein an opticalimage may be analyzed by conversion and scansion into a train ofelectrical signals. Such eicient image analysis tube, system and method.

My invention possesses numerous other objects and features of advantage,some of which, together With the foregoing, will be set forth in thefollowing description of specific apparatus embodying and utilizing mynovel method. It is therefore to be understood that my method isapplicable to other apparatus, and that I do not limit myself, in anyway, to the apparatus of the present application, as I may adopt variousother apparatus embodiments, utilizing the method, within the scope ofthe appended claims.

Referring to the drawing:

Fig. 1 is a longitudinal sectional View of one preferred form of tubeinvolved in practicing the method of my invention, together with aschematic circuit .for operation of said tube. Y

Fig. 2 is a cross sectional View of a control element utilized in Fig.1.

In the preferred form shown, an envelope I is provided at one endthereof With a transparent window 2 through which an optical image maybe focused by means of lens 4 onto a translucent photoelectric cathodey5 deposited on the inner wall of the envelope back of window 2.Obviously, a sensitized screen within the envelope is fully equivalentto the coating.

Within the tube, and spaced from cathode 5,'is a relativelyopen meshaccelerating electrode 6, this accelerating electrode preferably havinga relatively large void-to-wire ratio. On the other side of acceleratingelectrode 6 away from photoelectric cathode 5 is a charge storage screenelectrode] of relatively ne mesh, such as, for example, a perforatedmetal screen having approximately 400 meshes per inch. This screen isuniformly coated with insulating material 9 on the side facing theaccelerating electrode 6 and photocathode 5. This insulating coating maybe formed in any well known and convenient manner, such as, for example,the deposition by sputtering or the like, on the side to be insulated.of

an oxidizable vmaterial which, when oxidized, y

changes to an insulating oxide. Aluminum may be used for this purpose,as a specific example, or insulating oxides in nely divided conditionmay be sprayed, when mixed with a volatile liquid, on the side using theinsulating coating. Other methods of deposiitng this insulating coatingwill be immediately apparent to those skilled in the art.

At the end of the envelope opposite the transparent-window 2, anelectron gun is positioned, preferably to emit its undeflected beamaxially of the envelope. One exampel of such gun is shown and comprisesa unipotential thermionic cathode I0 heated by heater filament II.Electrons emitted from cathode I0 are accelerated through a centralbore, not shown, in a gun anode I2, as is well known in the art.Surrounding the greater `portion of the path between the gun anode I2and screens B and I is a conductive envelope wall coating I 4electrically connected with anode I through link I5.

'Ihe above description covers all of the fundamental electrode structurenecessary to practice my invention. One preferred circuit for operationof the tube is also shown, and in this circuit gun anode I2 and wallcoating I4 `are grounded through ground connection IS. Heater filament II is connected to the usual heater current source I1, and the thermioniccathode III is connected to the negative side of a D.C. anode source I8,the positive side of which is connected to ground. Photoelectric cathode5 is connected to the negative end of a D.-C. cathode source I9, thepositive end of which is grounded. Both anode source i8 and cathodesource I9 may have voltages for a specific tube in the neighborhood ofseveral hundred volts, but I prefer that the potential of thermioniccathode I0 be slightly more negative than that of photocathode 5 by anamount on the order of several Volts, say between 5 and l0 volts. Thus,in one specific tube it may be preferred to operate the device withthermionic cathode I0 at a potential of 405 volts and photocathode 5 at400 volts.

Storagev screen 'I is connected directly to ground. The acceleratingelectrode 6 is connected through output resistor 20, to potentiometercircuit 2I, the negative end of the potentiometer source 22 beingconnected to ground. Thus, accelerating electrode 6 may be held at apotential positive with respect to ground by several volts. The outputof the tube is taken from signal connection 24 connected directly toaccelerating electrode 6.

The cathode ray beam emitted from electron gun anode l2 is scannedacross storage screen 1 in two directions by means of scanning coils 25and 26 connected to scanning current generators .21 and 28,respectively. Obviously, however, electrostatic scansion may be utilizedif desired.

In operation, the optical image of the scene to be analyzed into a trainof signals is focused upon photocathode 5, thereby causingphotoelectrons to be emitted. It is then desired to produce an electronimage in the plane of the storage screen 1. As is well known in the art,this may be accomplished in different ways; iirst, lby spacingelectrodes 5, S; and 1 at'dis'- tances on the order of between 1/2A to 1inch apart 'in specific tubes, and then providing a magnetic focusingeld generated by a focusing coil surrounding that portion of the tubeadjacent the periphery of electrodes 5, 5 'and 1, 'or these sameelectrodes Ican be spaced atdistances on the order of le inch apartandthe electrostatic iields alone will focus the electron image. Suchexpedients are well known in, the art and are deemed fully equivalent.

The photoelectrons emitted from photoelectric cathode 5 under theinuence of the optical image are therefore accelerated by the higherpotential on accelerating screen E, pass through this relatively openscreen, and travel toward storage screen 1. In the plane of storagescreen 1 an electron image is formed, as described above, correspondingto the optical image. 'This electron image will arrive at the insulatingsurface 9 of storage screen 1 with a velocity corresponding to thepotential difference between photocathode 5 and the metallic portion ofstorage screen 1 which may be, as above described, 400 volts. Thiselectron velocity is sufcient to produce from the insulating materialswhich may be utilized to form the insulating coating, secondaryelectrons at a ratio higher than unity. These emitted secondaryelectrons are then accelerated toward accelerating electrode 6 which isat a voltage higher than storage screen 1 by the amount determined bythe adjustment of potentiometer circuit 2|, and these secondaryelectrons are therefore collected by accelerating electrode 6. Thiscollected electron current constitutes the so-called D.C. component ofthe picture signals, and corresponds to the average brightness of theoptical image.

The secondary electrons leaving the insulating portion 9 of the storagescreen leave-elementary areas of the insulating portion positivelycharged by an amount comparable to the number of secondary electronsemitted, which latter are in turn dependent upon the nurrrber ofphotoelectrons impacting this insulating material. This impacting numberis again a function ofI the brilliancy of the optical image ondifferentelementary areas of the photoelectric cathode. Thus, a positivecharge image is produced on the insulating surface 9 corresponding inall respects with the optical image.

The metallic side of the storage screen 1 facing the gun anode l2 isthen scanned by-the beam issuingfrom the gun anode l2, which is confinedto have a cross section of elemental area, and electrons inthis beam,under the voltages used, have sufficient velocity also to producesecondary emission at aratio greater than unity upon irnpact with themetallic side of screen 1. In this respect it is to bedistinctly'understood that if it appears desirable, the metallic side ofscreen 1 may be subjected to special treatments `well known in the artin order to increase the ratio of secondary emission. Under normalcircumstances, however, materials can be chosen for the metal of screen1 which will have a suiciently high ratio within the voltage rangesused.

The secondary electrons emitted on the metallic side of. the4 storagescreen 1 upon impact of the scanning beam issuingK from` the anode l2may be said to form a small cloud of electrons about the point ofimpact. The secondary electrons will of course have low emissionvelocities, and are therefore subjected to, and come under, theinfluence of the positive charges on the insulating side 9, the lattercharges providing a potential 'gradient pulling secondary electronsthrough the apertures in the screen, and the number of secondaryelectrons of the cloud pulled through is of course proportional to thepositive charge on the insulating side 9 at the point of secondaryproduction on the metallic side. Some` of the secondary electronsvpulled through will neutralize the. positiv/eY charges on the insulatingmaterial, while the majority of them will be accelerated toward screen 6and be collected thereby, Thus, in addition to the so-called D.C'.component` already described, there will be collected on acceleratingelectrode 6 a varying electron current constituting, through outputlead; 24, a train of picture signals.

Aportion ofthe scanning beam will of course pass throughthe openings instorage` screen 1 without impacting it. The majority of these beamprimary electrons, however, pass directly through accelerating electrode6 and are collected by photocathode 5, because, as was above stated,this photocathode is held at a potential a Yfew volts higher thanA gunanode I2. Therefore, except for the minute number of primary beamelectrons which may be actually intercepted by the wirev of acceleratingelectrode 6, noneof theprimary electrons from gun anode l2y will enterthe picture signal. Consequently, it is possible to reduce the picturecurrent. substantially to "zero `in absence of lightin the` opticalimage. A

It has been found in practice that a positive charge on the order of 1volt on the insulating surface 9-of the storage screen maybe sufficientto'saturate the secondary emission from the metallic side. It willtherefore be preferable to choose the potential of acceleratingelectrode 6 so that equilibrium, i. e., the number of arrivingphotoelectrons, to Abe equal to that portionof the secondariesproduced-by photoelectrons (which portion is collected, of course, byelectrode 6), is reached when a positive charge of l` volt` hasaccumulated on the insulating side 9? for the brightestspots of the.image. The-time in which this equilibrium can be. "reached" is made ltoap proximate the time between scansions.

Therefore, under these circumstancesr I- may adjust. the potentialdifferencefbetwenf photocathode 5' andstorage screen 1 to! a'valu'e. forwhich equilibrium may be reached at a positive charge of 1 volt onV the`insulating Y coating', and then choose the lightintensity ofrthebrilghtest portions of the optical imagesto be analyzed to have sucha value thatfthe time required. to accumulate thispositivecharge of .1volt-on the insulating side Slis approximately` equal to the timebetween scansions.

It should be distinctly.A understood, however. that the value of 1volt'herewith given is alvalue arrived atinconjunction Withlthe othervoltage` values herein` mentioned, and has been arrived at byobservation. Therefore, I do not Wish to limit the scope of my inventionin any way to the volt-age values herewith disclosed, inasmuch as thesevalues may be raised and lowered, and the best operating conditions withchanged voltages arrived at, by observation during operation, whichprocedure is well known to all those skilled in the operation of imageanalysis tubes.

I claim:

l. The method of generating a television picture signal which comprisesforming a charge image in accordance with an optical image to betransmitted, successively creating a plurality of secondary electronsadjacent successive elemental areas of said charge image, modulating thestream of said secondary electrons in accordance with the charges ofsaid elemental areas, utilizing a portion of said modul-ated stream ofsecondary electrons to neutralize thecharges of successive element-alareas of said charge image, and collecting the remainder of saidmodulated stream of lsecondary electrons to develop a picture signal.

2. The method of generating a television picture signal which comprisesproducing a cloudv of secondary electrons representing a virtual cathodein space, moving said cloud in a plane in accordance with a scanningpattern, creating a charge image representative of an optical image in aplane closely adjacent the plane of said moving cloud, drawing electronsfrom said cloud in accordance with the charges of said charge image, andcollecting the electrons drawn from said cloud to develop a picturesignal.

3. The method of generating a television picture signal which comprisesgenerating a beam of primary electrons, utilizing said beam to produce acloud of secondary electrons representing a virtual cathode in space,moving said cloud in a plane by scanningsaid beam in accordance With ascanning pattern, creating a charge image representative of an opticalimage in a plane closely adjacent the plane of said moving c loud,drawing electrons from said cloud in accordance with the charges of saidcharge image, and collecting the electrons drawn from said cloud todevelop a picture signal.

4. 'I'he method of generating a television picture signal whichcomprises producing a cloud of secondary electrons representing avirtual cathode in space, moving said cloud in a plane in accordancewith a scanning pattern, creating a charge image representative of anoptical image in a plane closely Iadjacent the plane of said movingcathode, drawing electrons from said cloud in accordance with thecharges of said charge image, utilizing a portion of the electrons drawnfrom said cloud to neutralize said charges, and collecting the remainderof the electrons drawn from said cloud to develop a picture signal.

5. A television picture signal generator comprising an envelopecontaining an apertured screen, one side of said screen being adapted tostore electrical charges, means for scanning the other side of saidscreen with an electron beam to liberate secondary electrons upon impacttherewith, means including a photoelectric cathode for producing acharge image on said rstnamed side of said screen representative of anoptical image to draw a portion of said secondary electrons through theapertures of said screen in accordance with the charges of said chargeimage, and means for collecting said portion of electrons to develop apicture signal.

6. A television picture signal generator, comprising an envelopecontaining an apertured screen, one side of said screen being adapted tostore electrical charges, means for scanning the other side of saidscreen with an electron beam to liberate secondary electrons upon impacttherewith, a photoelectrilc cathode adapted to receive an optical imageand to transmit a corresponding electron image of such velocity uponsaid first-named side of said screen to produce by secondary emission acharge image representative of said optical image to draw a portion ofsaid secondary electrons through the apertures of said screen inaccordance with the charges of said pharge image, and means forcollecting said portion of electrons to develop a picture signal.

7. A television picture signal generator comprising an envelopecontaining an apertured screen, one side of said screen being adapted tostore electrical charges, means for scanning the other side of saidscreen with an electron beam to liberate secondary electrons upon impacttherewith, a photoelectric cathode adapted to receive an optical imageand to transmit a corresponding electron image oi such velocity uponsaid first-named side of said screen to produce by secondary emission acharge image representative of an optical image to draw a portion ofsaid secondary electrons through the apertures of said screen inaccordance with the charges of said charge image, and means disposedbetween said photoelelctric cathode and said apertured screen forcollecting said portion of electrons to develop a picture signal.

PHILO T. FARNSWQRTH.

